Ultrasonic transducers incorporate one or more piezoelectric vibrators which are electrically connected to a pulsing-receiving unit in the form of an ultrasonic test unit. The piezoelectric transducer converts an electrical pulse from the pulsing unit and converts the electrical signal to a mechanical vibration which is transmitted through a material such as a metal to which it is coupled. The piezoelectric material has the ability to receive a mechanical vibration from the material to which it is coupled and convert it to an electrical pulse which is sent to the receiving unit. By tracking the time difference between the transmission of the electrical pulse and the receipt of the electrical signal and measuring the amplitude of the received wave, various characteristics of the material can be determined. The mechanical pulse is generally in the frequency range of about 0.5 MHz to about 25 MHz, so it is referred to as an ultrasonic wave from which the equipment derives its name. Thus, for example ultrasonic testing can be used to determine material thickness or the presence and size of imperfections within a material.
Ultrasonic transducers can be used in pairs of send only and receive only units. However, frequently the transducers are transceivers that both send and receive the pulses. The transducer may be a single element or a single transducer may be comprised of a plurality of ceramic elements. This invention is broadly directed to the transducer or probe that is comprised of a ceramic element or elements and improved methods of manufacturing such transducers. Such transducers that utilize the composite ceramic include single element transducers, dual element transducers and arrayed (phased array) transducers.
These transducers currently are produced by providing a single-piece ceramic material, such as a lead-zirconate-titanate (PZT) ceramic. This ceramic is processed to produce a plurality of spaced columns/posts or planes of preselected side projecting from a solid piece of the ceramic which is unaffected by the processing. This unaffected solid piece of ceramic is referred to as the ceramic backbone. This invention is more narrowly directed to improvements in processing of the ceramic element that is used in an ultrasonic probe, the probe including not only the ceramic, but also the matching layers, the backing, the case and the connectors.
After the plurality of spaced columns or spaced planes, also referred to as a diced ceramic, has been formed, the spacing between the columns or planes is filled with an epoxy polymer. Sufficient epoxy polymer is applied to form a continuous layer of epoxy overlying the diced ceramic and opposite the ceramic backbone.
The ceramic backbone is then removed by grinding. To assure complete removal of the ceramic backbone, the ceramic removal operation extends below the backbone and slightly into the diced ceramic, removing a small portion of each post or plane. This is not significant, as it is important to maintain a flat surface with a smooth surface finish. The surface standard for parallelism is 0.0002″ after grinding, and the surface finish is about 35,000 Angstrom units or smoother, typically between about 15,000 to about 35,000 Angstrom units. After the backbone has been removed, the workpiece is flipped over and the epoxy polymer is removed by grinding. Again, some small portion of each post may be removed, but it is important to maintain a flat surface with a smooth surface finish. It is not important whether the epoxy polymer or the ceramic backbone is removed first, although the processing is somewhat easier if the ceramic backbone is removed first. At this point in the processing, the workpiece comprises a plurality of ceramic posts embedded in an epoxy polymer. Both sides of the workpiece are then finish ground. After finish grinding, the ceramic posts are depressed typically from about 15,000 to about 30,000 Angstroms below the epoxy polymer. The depression of the posts can be reduced to 2000 Angstroms below the surface of the epoxy by an optional polishing step. A cross-section of a prior art multi-arrayed transducer after an optional polishing operation is shown in FIG. 1 with a layer of plating 32 applied over its surface, depicting the ceramic posts 12 lying below the surface of the epoxy 24.
The ceramic is cleaned in an ultrasonic cleaner to remove any damaged ceramic. The power setting of the cleaners are adjustable, and the power setting is adjusted to a level at which plating on posts is not removed is not removed from the posts during cleaning. After cleaning, the ceramic is rinsed followed by plasma cleaning, the ceramic workpiece is sputter plated, and the plating is tested for adhesion. The ceramic workpiece is then dice-deactivated and poled to activate the ceramic.
While this process can produce an effective transducer, there are problems associated with such transducers. These problems are associated with ceramic posts or planes being depressed below the surface of the epoxy. The sputter plating process provides a very thin plating over the surface. Total plating thickness is about 15000 Angstroms, which is applied by a line of sight process. Because the ceramic posts are depressed below the surface of the epoxy polymer, it is possible that the sputtering process may not provide a uniform coating of the surface, particularly along the perpendicular surfaces extending between the parallel planes of epoxy polymer and ceramic material. In addition, since the sputter plating operation is performed at temperatures of about 120° C. (about 250° F.), the epoxy is free to expand unrestrained above the ceramic posts or planes. Even though this expansion is small, because of the thinness of the plating deposited by the sputter plating process, it can be sufficient to damage the thin plating extending in the vertical direction along the epoxy polymer between the ceramic posts and the horizontal surface of the epoxy, causing poor performance of the ceramic, such as low capacitance. After sputtering, the ceramic is dice-deactivated and poled to activate the ceramic. The temperatures for poling can be in the range as high as about 100–110° C. After poling, contacts are soldered to the plating.
Another problem with this configuration is that the depressed ceramic is difficult to solder. As a result, the solder heat is borne by the epoxy during the soldering process, causing it to expand, and further increasing the possibility that the thin plating may fracture, thereby causing bad solder connections.
A transducer with a plurality of elements formed from a ceramic, which elements are not depressed below the polymer, would overcome many of the difficulties associated with the prior art transducers described above, but such a transducer and a method for fabricating such a transducer is heretofore unknown to the art.